Alkylation reactions are used to add an alkyl group to a molecule that is to be alkylated. Alkylation reactions are extensively used in the petroleum industry to produce medium- or large-mass hydrocarbons from smaller molecules. The alkylation reaction is typically used to alkylate a high vapor pressure paraffin (an alkane) with an olefin (an alkene) to produce a low vapor pressure, high-octane gasoline blend. This gasoline blend is a clean gasoline blend stream and provides 13%-15% of gasoline demand in the United States. One important alkylation reaction in the petroleum industry is the alkylation of isobutane with butene to produce isooctane. Currently, the alkylation reaction is catalyzed with a concentrated liquid mineral acid, such as hydrofluoric acid or sulfuric acid. However, large acid volumes and acid-oil sludges are produced by the mineral acid-catalyzed reaction, raising safety and environmental concerns. Disposal of the acid-oil sludge is subject to stringent environmental regulations, which adds considerable expense to the alkylation reaction. In addition, leakage of the acid, either as a liquid or a solid, is another significant safety concern.
To reduce the environmental concerns, solid alkylation catalysts have been used to catalyze the reaction. However, these solid alkylation catalysts deactivate rapidly due to the accumulation of hydrocarbon species (also known as coke) on a surface of the solid alkylation catalyst. Coking of the solid alkylation catalyst is caused by side reactions that involve acid-catalyzed polymerization and cyclization of the reactants and/or reaction products, which produces high molecular-weight compounds that undergo extensive dehydrogenation, aromatization, and further condensation. As the hydrocarbon species accumulate, they deactivate the solid alkylation catalyst and decrease its ability to effectively catalyze the alkylation reaction. These hydrocarbon species are difficult to remove from the solid alkylation catalyst. The solid alkylation catalyst is typically regenerated or reactivated by burning off or gasifying the coke compounds. However, since these regeneration processes are oxidative and damage activity of the solid alkylation catalyst, the solid alkylation catalyst is only capable of being regenerated a few times. The rapid deactivation of the solid acid catalyst also produces large volumes of the solid alkylation catalyst that must be discarded, making the alkylation reaction and subsequent regeneration of the solid alkylation catalyst economically and environmentally unacceptable.
U.S. Pat. No. 6,579,821 to Ginosar et al. (the “Ginosar '821 patent”), the disclosure of which is incorporated by reference herein, discloses the use of a supercritical fluid to regenerate the alkylation catalyst. The supercritical fluid is isobutane, isopentane, 2,3-dimethylbutane, 2-methylpentane, 3-methylpentane, 2,3-dimethylpentane, 2,4-dimethylpentane, 2-methylhexane, 3-methylhexane, 2,3-dimethylhexane, 2,4-dimethylhexane, 2,5-dimethylhexane, 3,4-dimethylhexane, or 2,3,4-trimethylhexane. Extractive properties of the supercritical fluid are used to remove the hydrocarbon species that are adsorbed onto the alkylation catalyst. As the supercritical fluid contacts the alkylation catalyst, the adsorbed hydrocarbon species are dissolved and removed. The supercritical fluid regeneration process enables the alkylation catalyst to be reactivated or regenerated more than fifty times.
While the supercritical fluid removes most of the hydrocarbon species, the supercritical fluid has limited ability to effectively remove condensed hydrocarbon species that are present on the alkylation catalyst. As used herein, the term “condensed hydrocarbon species” refers to a hydrocarbon compound(s) that is formed by an alkylation or oligomerization reaction, followed by cyclization and/or dehydrogenation of the hydrocarbon species. As the condensed hydrocarbon species accumulate on the alkylation catalyst, they limit the extent of regeneration and the economics of using the supercritical fluid regeneration process.
Surface modification of catalysts used in various types of reactions is known in the art to increase performance (activity) or to decrease deactivation of the catalyst. As disclosed in Liu et al., “Surface modification of zeolite Y and mechanism for reducing naphtha olefin formation in catalytic cracking reaction,” App. Catal. A: General, 264:225-228 (2004), a surface of zeolite Y is modified with a rare earth compound and a phosphorus compound to improve acidity density and strength in pores of the zeolite Y and to decrease surface acidity density of the zeolite Y. As disclosed in Inui et al., “Effect of Modification of Acid Sites Located on the External Surface of a Gallium-Silicate Crystalline Catalyst on Reducing Coke Deposit in Paraffin Aromatization,” Ind. Eng. Chem. Res., 36:4827-4831 (1997), acid sites on an external surface of MFI-type gallium-silicate crystals are selectively modified with cerium oxide. The cerium oxide is used to neutralize the acid sites, moderating deactivation of the MFI-type gallium-silicate crystals and reducing deposition of coke compounds on the MFI-type gallium-silicate crystals. Chen et al., “Effects of surface modification on coking, deactivation and para-selectivity of H-ZSM-5 zeolites during ethylbenzene disproportionation,” J. Molec. Catal. A: Chemical, 181:41-55 (2002), discloses modifying a surface of an H-ZSM-5 zeolite by silica chemical vapor deposition (“Si-CVD”) or by a combination of lepidine adsorption and Si-CVD to improve the performance of the H-ZSM-5 zeolite.
U.S. Pat. No. 6,440,886 to Gajda et al. discloses a surface-modified beta zeolite that has decreased deactivation. The beta zeolite is used to catalyze alkylation or transalkylation of an aromatic compound. The surface of the beta zeolite is modified by removing strong acid sites, such as by converting the strong acid sites to weaker acid sites that are ineffective or less effective. To modify the catalyst, the beta zeolite is exposed to a strong mineral acid, such as nitric acid, sulfuric acid, phosphoric acid, or hydrochloric acid. The beta zeolite is then calcined at a temperature ranging from 550° C. to 700° C. In U.S. Pat. No. 5,043,307 to Bowes et al., a modified aluminosilicate zeolite is disclosed. The aluminosilicate zeolite is modified by steaming to decompose template material and to remove zeolitic aluminum. The zeolite is then contacted with a dealuminizing agent to form a water-soluble aluminum complex. U.S. Pat. No. 5,237,120 to Haag et al. discloses isomerizing a terminal double bond olefin-containing organic feedstock to an internal double bond olefin. The isomerization reaction is performed using a surface-modified double bond isomerization catalyst that is partially deactivated for acid catalyzed reactions. The double bond isomerization catalyst is modified by chemisorbing a surface-deactivating agent to the surface. The surface-deactivating agent is an amine, phosphine, phenol, polynuclear hydrocarbon, cationic dye, or organic silicon compound.